Blood pumps have been commonly used to provide mechanical support or assistance to the left ventricles of patients. Typically, the left ventricle is responsible for pumping blood into the aorta and throughout a majority of the patient's body. Ventricular assistance has been previously been provided by an implanted blood pump, such as the Ventrassist™ rotary blood pump described in U.S. Pat. No. 6,227,797—Watterson et al.
These blood pumps generally pump blood in parallel to the normal circulatory system by removing blood directly from the left ventricle and pumping into a portion of the aorta. Generally when such a blood pump is implanted, blood may flow or be pumped by both the left ventricle and the blood pump.
The speed of the implanted blood pump is preferably carefully monitored and controlled. Preferably, the pump and the respective controller should be able to adapt to changes in physiological demand for blood of the patient's body. Preferably, the blood pump should not be allowed to run so fast that the pump causes a suction event whereby the pump receives less blood flow and the contractile properties of the ventricle are effectively lost or diminished. In severe situations of a suction event, the ventricle wall is pulled over an inflow of the blood pump and may completely occlude blood flow.
In the past, if an implanted blood pump pumps blood at a too high rate, when compared to the left ventricle, the heart may experience arrhythmias. Additionally, if the pump is operating at a relative speed that is too low, when compared to the left ventricle, the patient may experience oedemas.
U.S. Pat. No. 6,949,066—Beamson et al discloses a pump control system for use with a centrifugal type blood pump of the kind used as a left ventricle assist device. The system includes a first sensor that detects at least one operational parameter of the pump; and a second sensor that detects and measures at least one physiological parameter of a patient implanted with the pump. This system fails to address situations wherein intermittent suction events are occurring to a patient implanted with a left ventricle assist device. Additionally, the addition of sensors to the system will add to its complexity and increase the likelihood of device failure.
U.S. Pat. No. 6,991,595—Burke et al describes an adaptive speed control for an implanted blood pump wherein the control is adapted to detect the onset of left ventricular collapse by deriving and monitoring a pulsatility index, and adjusting the pump speed to maintain the pulsatility index at a pump set-point. The pulsatility set-point is decreased incrementally when the onset of ventricular collapse has not been detected for a predetermined period of time. Experimentally, it has been found that pulsatility index is the most or preferred physiological characteristic for determining the imminence of a suction event.
U.S. Pat. No. 6,783,328—Lucke et al discloses a pump control system that monitoring flow and/or pressure of the pump output and decides whether the flow or pressure exceeds a critical level. If the critical level is exceeded the control system reduces the pumping speed by reducing the pump set-point. This system relies on the expectation that the all of the pressure and/or flow experienced by blood at the pump outlet is the solely the output of the pump, this system ignores other pumping elements such as the natural heart which is still capable of supplying a proportion of flow and pressure. The system also only detects whether the threshold flow or pressure values have been exceeded and does not detect or determine any suction events.
U.S. Pat. No. 5,888,242—Antaki discloses an automatic speed control system for implanted blood pump wherein the speed of the pump is incrementally increased and when the system detects the imminence of a suction event occurring to the left ventricle, the system decreases the pump speed by a predetermined amount. A disadvantage with this system is that when the system detects the imminence of a suction event, the system slows the pump and then gradually increases the speed until the imminence is detected again. Hence the system continually repeats the error despite its detection and this may be dangerous for the patient.
U.S. Pat. No. 6,066,086—Antaki discloses a further automatic speed control system for use with an implanted cardiac assist blood pump wherein the system operates in a manner to prevent the opening of aortic valve during the operation of the heart, once it has been implanted with a left ventricle assist device. Experimentally, and to the contrary the inventors of the invention described within the present specification have found that it is preferable to allow the aortic valve to open and close during the operation of a left ventricular assist device. Furthermore the system described in the disclosure of U.S. Pat. No. 6,066,086 does not measure or predict suction events.
U.S. Pat. No. 6,623,420—Reich et al discloses a system wherein an implanted blood pump is connected to a single blood pressure detector which is positioned in the inflow of the pump. The detector continuously detects blood pressure within the cannula and compares the detected blood pressure to a tabulated predicted blood pressure. The system then adjusts the pump speed to minimize the difference between the detected value and the predicted value. Suction events are not directly avoided by this system.
U.S. Pat. No. 6,443,983—Nagyszalancy et al discloses a pump speed control system wherein blood flow and pressure are both detected and measured and then used in a feedback to control speeds of two blood pumps. The system requires measured values for both flow and pressure to function and as such the system requires the additional implantation or use of sensors in the blood path, which is preferably avoided as they may lead to points of blood flow stagnation or blood clotting.
International Patent Publication No. WO 03/057013—Morello describes a control system for use with a blood pump. The system described within this disclosure is primarily reliant on a flow sensor included within the system. Generally, it has been found experimentally that sensors may increase the likelihood of device malfunction and/or reduce the overall biocompatibility of the system.
International Patent Publication No. WO 04/028593—Ayre describes a further control system for a rotary blood pump. However, this system generally includes a type of pressure sensor for determining blood pressure. It is generally desirable to remove sensors that may contact the blood.
The present invention aims to or at least address or ameliorate one or more of the disadvantages associated with the above mentioned prior art.